This application claims priority under 35 U.S.C. xc2xa7 xc2xa7 119 and/or 365 to the Japanese Patent Application Nos. 2002-191299 and 2003-130155 filed in Japan on Jun. 28, 2002 and May 8, 2003, respectively, the entire content of which is herein incorporated by reference.
The present invention relates to a non-heat treated crankshaft that uses xe2x80x9cnon-heat treated steel that exhibits, even when thermal refining after hot forging is omitted, excellent strength, excellent low temperature toughness and excellent wear resistancexe2x80x9d and is preferable for use in general-purpose engines for such as lawn mowers, electric generators, snow blowers, pumps, and outboard motors.
Machine structural steel parts, that have been so far used in automobiles and construction machinery, have been generally produced according to a method in which the steel for machine structural use (for instance, S48C and so on that are carbon steels for machine structural use defined in JIS G 4051) is formed into necessary part shapes by applying hot forging followed by applying quenching and tempering heat-treatment (so-called thermal refining) so as to endow necessary strength and toughness.
However, in recent years, in response to demands for cost reduction and lead-time reduction, development of xe2x80x9cnon-heat treated steels capable of being used as hot forged without applying thermal refiningxe2x80x9d have been forwarded, and now, the non-heat treated steels have rapidly expanded their applications and now focus on fields such as the automobiles and construction machinery.
As the demand for the non-heat treated steels spreads, the demands for higher performance non-heat treated steels and the cost reduction thereof, have become stronger and high-toughness non-heat treated steels, that can ensure excellent toughness even without applying the thermal refining and wear resistant non-heat treated steels that exhibit excellent wear resistance, even without applying surface hardening such as induction hardening and so on, have been proposed.
For instance, in Japanese laid-open patent application (JP Kokai) H08-120342, xe2x80x9cProduction of non-heat treated hot forged steel part having high strength and high toughnessxe2x80x9d is disclosed. According to the production method, a billet thereto, either a specified ratio of C, Mn, P and N is added followed by further adding one kind or more of V, Ti and Nb, or a specified ratio of C, Mn, Cr, V and B is added followed by further adding one kind or more of Ni, Cu and Mo is subjected to hot rolling, thereby steel whose austenite grain size number is controlled in a particular range is manufactured, thereafter the steel is subjected to hot forging controlled in a heating temperature, a temperature raising speed and a heating and holding period, and thereby non-heat treated hot forged parts, with high strength and high toughness, and having a microstructure mainly composing of a ferrite-pearlite, is produced.
Furthermore, in JP Kokai H10-277705, xe2x80x9cProduction of non-heat treated bar steel for high toughness hot forgingxe2x80x9d is disclosed. According to the production method, when a molten steel, whose carbon equivalent (Ceq.) is in the range of 0.7 to 1.30 and that contains a specified ratio of C, Si, Mn, Cr, V, Ti, N and Al, is continuously cast, a hot billet is formed under controlled cooling conditions, followed by heating and rolling under particular conditions, and thereby a bar steel is obtained.
Furthermore, in JP Kokai 2000-265242, xe2x80x9cNon-heat treated steel for hot forging excellent in wear resistancexe2x80x9d is proposed. In the steel, C is contained in a range of 0.40 to 0.70% (hereinafter xe2x80x9c%xe2x80x9d that shows a component ratio means xe2x80x9c% by massxe2x80x9d), a specified ratio of Si, Mn, Cr, Al and N is contained, or as needs arise one kind or more of Pb, S, Te, Ca and Bi, that are machinability improvement elements, are further contained, a composition after the hot forging is a ferrite-pearlite microstructure, and an area ratio of pro-eutectoid ferrite is 10% or less.
Furthermore, in JP Kokai 2000-328193, xe2x80x9cNon-refining steel for hot forging excellent in wear resistancexe2x80x9d is disclosed. The hot forging non-heat treated steel contains C, Si and Mn at a specified ratio, or, as needs arise, further contains one kind or more of V, Nb and Ti, that are carbide- and nitride-forming elements, Cr that is a hardenability improvement element, Al that is an element that makes austenite grains finer and S, Pb, Zr, Ca, Te and Bi that are machinability improvement elements, and has a controlled amount of oxide inclusions, having a grain size larger than a particular value.
Working machines such as lawn mowers and electric generators, those are nowadays spreading into homes, are provided with a general-purpose engine called an xe2x80x9cengine for working machinexe2x80x9d. In such an engine for working machine, a structure in which a working member, such as a cutter blade for use in lawn mower, is directly fixed to the crankshaft of the operating engine is frequently adopted, and it is inevitable that the impact load of the working member is directly transmitted to the crankshaft.
For instance, FIG. 1 is a schematic explanatory lateral sectional view showing an example of an internal structure of an engine in a lawn mower. A lawn mower cutter blade [2] as a working member is a solid body connected to a shaft end of the crankshaft [1] and the impact load to the lawn mower blade [2] is directly transmitted to the crankshaft [1].
Accordingly, in the crankshafts that are supplied for such applications, in addition to general demands, such as the strength and the wear resistance, the toughness in an application environment temperature region of the working machine, in particular very excellent low temperature toughness that can withstand the use even in cold regions, is demanded.
Accordingly, although many proposals relating to non-heat treated steels have been disclosed, it is generally known that as for machine structural steel parts, such as the crankshafts of engine for working machine that are required to be excellent in the low temperature toughness, in addition to the strength and the wear resistance, as identical as so far, the steel for machine structural use such as the S48C is hot forged, followed by applying the thermal refining due to quenching and tempering heat-treatment, and thereby predetermined low temperature toughness, wear resistance and strength are endowed.
The reason for the above is as follows:
As for the machine structural steel parts such as mentioned above, when existing non-heat treated steel is used as hot forged without thermal refining due to quenching and tempering heat-treatment, sufficiently satisfying low temperature toughness and wear resistance cannot be simultaneously obtained.
That is, since the low temperature toughness and the wear resistance of the steel are in a conflicting relationship, the non-heat treated steels that have been so far proposed are difficult to enhance both of the above properties simultaneously. For instance, all of the non-heat treated steels disclosed in the above-mentioned JP Kokai H08-120342, JP Kokai H10-277705, JP Kokai 2000-265242 or JP Kokai 2000-328193, in a state where the thermal refining is omitted, can neither ensure the low temperature toughness and the wear resistance needed to satisfy crankshafts of such as engines for working machine, nor sufficiently cope with demands for cost reduction and lead time reduction, due to the omission of the thermal refining.
The present invention intends to provide a non-heat treated crankshaft that can exhibit, even when thermal refining after hot forging is omitted, excellent strength, excellent wear resistance and excellent toughness.
The gist of the present invention is as follows:
(I) A non-heat treated crankshaft which comprises, in % by mass, C: 0.30 to 0.35%, Si: 0.40 to 0.80%, Mn: 1.00 to 2.00%, S: 0.040 to 0.080%, Cr: 0.10 to 0.30%, and V: 0.05 to 0.20%, with the balance being Fe and impurities, and of which the microstructure is a ferrite-pearlite microstructure whose ferrite fraction xcex1 is in the range of 0.20 to 0.60 and average pearlite grain size Dp is 0.08 mm or less, and the value of Fn1 defined by the following equation (1) being 0.73 or more, the value of Fn2 defined by the following equation (2) being 1.20 or more and the value of Fn3 defined by the following equation (3) being 0.64 or more:
Fn1=C+0.10Si+0.20Mnxe2x88x920.71S+0.23Cr+1.65Vxe2x80x83xe2x80x83(1),
Fn2=1.4xe2x88x921.1xc3x97(1xe2x88x92xcex1)+0.16Dpxe2x88x92xc2xdxe2x80x83xe2x80x83(2),
Fn3=(1xe2x88x92xcex1)+xcex1xc3x97(0.3Si+0.8V)xe2x80x83xe2x80x83(3),
wherein, in the above equations (1) and (3), the symbols of the elements represent the contents, in mass %, of the elements in the steel.
Here, the xe2x80x9cferrite-pearlite microstructurexe2x80x9d means a mixed microstructure of a ferrite phase and a pearlite phase.
Furthermore, the xe2x80x9cferrite fraction xcex1xe2x80x9d means an area ratio of the ferrite microstructure in the ferrite-pearlite microstructure confirmed by optical microscope observation, and xe2x80x9cthe ferrite fraction xcex1 is in the range of 0.20 to 0.60xe2x80x9d means that a ratio of the ferrite phase in an observed field is in the range of 20% to 60%.
The xe2x80x9caverage pearlite grain size Dpxe2x80x9d means an arithmetic mean value of dimensions of grains (grain size) of the pearlite phase confirmed with the optical microscope, and xe2x80x9cgrain size of the pearlitexe2x80x9d means, so called xe2x80x9csize of pearlite colonyxe2x80x9d.
In order to further improve the toughness of the non-heat treated crankshaft described in the (I), in lieu of part of Fe, one or more elements selected from among Ti: 0.050% or less, Zr: 0.050% or less and Nb: 0.050% or less may be contained.
In order to improve the machinability of the non-heat treated crankshaft described in the (I), in lieu of part of Fe, one or both elements of Pb: 0.30% or less and Ca: 0.010% or less may be contained.
In order to further improve the toughness of the non-heat treated crankshaft described in the (I) and at the same time enhance the machinability thereof, in lieu of part of Fe, one or more elements selected from among Ti: 0.050% or less, Zr: 0.050% or less and Nb: 0.050% or less, and one or both elements of Pb: 0.30% or less and Ca: 0.010% or less may be contained.
As mentioned above, the symbols of the elements in the equations (1) and (3) respect the contents (% by mass) of the elements in the steel.